2. INTERESTING LONG-LIVED GALACTIC CHRONOMETERS

The identification of 232Th, 235U, and
238U as the most useful, and currently most
reliable, nuclear chronometers follows from several considerations. For
our present purposes,
we wish to consider long lived chronometers, with lifetimes comparable
to or greater than the age of the Galaxy we seek to measure.
Table 1 presents a compilation of such
potentially interesting galactic chronometers and identifies the
nucleosynthesis mechanisms
by which they are believed to be formed. Note particularly the potential
additional long
lived (1/2 >
109yr) galactic chronometers 187Re,
40K, 87Rb, 138La, 147Sm,
176Lu, that we
have chosen, for a variety of reasons, not to emphasize in our
discussion. 40K cannot yet
be utilized as a useful chronometer, since neither the nuclear process
by which it is formed
nor its level of production is known to a reasonable degree of
certainty. The effective
use of 87Rb is limited, among other things, by the fact that
both the s-process and the
r-process can contribute significantly to its production; the same is
true of 147Sm. 138La
is a p-process nucleus, for which no reliable determination of its
production history is yet
possible. 176Lu is an s-process nucleus for which, again, an
accurate quantitative measure
of its production history is lacking. Finally, the case of
187Re deserves special mention.
The long lifetime of 187Re makes the
187Re-187Os chronometer pair a particularly
attractive choice for dating galactic nucleosynthesis
(Clayton 1964).
A major difficulty here is
the fact that 187Os is also produced in the s-process. The
uncertainties introduced by
the subtraction of the s-process contribution to isolate the
cosmo-radiogenic component
(Bao & Kappeler 1987)
are significant. Further complications are associated with the
fact that the beta decay rate of 187Re in stellar
environments is sensitive to temperature
(Clayton 1969;
Cosner & Truran 1981;
Arnould et al. 1984).
For the purposes of this
paper, we will then restrict ourselves to the
232Th-235U and 235U-238U
chronometer pairs.